The present invention relates to a hermetic motor compressor unit, particularly to such a unit which is intended for use in small capacity applications, such as small refrigerators.
One of the primary concerns in designing refrigeration compressors for use in small capacity applications is that of minimizing the overall size of the unit without sacrificing efficiency or the capacity which is required. A further design consideration is that of minimizing the number of parts required and the assembly time. This is particularly important in small compressors because the manufacturing volume of such compressors is normally quite high and even small savings in material and labor reaches considerable proportions when high production levels are attained.
One of the assembly operations performed in manufacturing such a compressor is that of assembling the connecting rod to the crankshaft and piston. Because the connecting rod articulates about the piston wrist pin only in directions transverse to the axis of the crankshaft, it is impossible, when using most conventional techniques, to insert the connecting rod over the end of the crankshaft when the connecting rod is attached to the piston. One technique for assembling the connecting rod to the crankshaft is the use of a split sleeve-type connecting rod wherein the sleeve halves are assembled around the crankshaft and secured together by means of bolts. The problem with this technique is that additional parts are required and there is a substantial amount of labor in assembling the connecting rod around the crankshaft. Furthermore, the split sleeve is a difficult part to manufacture due to the necessity for accurate machining of the mating surfaces thereof.
A further solution to the problem would be to initially install the piston and connecting rod assembly into the crankcase and then insert the crankshaft through the open loop bearing end of the connecting rod. This solution is not feasible in the case of the compressor in question, however, wherein the crankshaft is disposed vertically and must have a relatively large bearing surface in contact with the supporting surface of the crankcase. This would require a correspondingly large opening in the connecting rod, which is not practical in very small compressors wherein the connecting rod is generally small. Although the connecting rod could be lengthened to accommodate the larger opening, this would increase the overall size of the compressor in the direction of the connecting rod. As mentioned earlier, minimizing the overall size of the unit is one of the design criteria of compressors of this type.
U.S. Pat. No. 3,903,752 discloses yet another solution to the problem of assembling the piston, connecting rod and crankshaft. The wrist pin and connecting rod form a unitary assembly, which is inserted into the cylinder through a slot in the sidewall thereof at the same time that the integral, open loop bearing end of the connecting rod is slipped over the end of the crankshaft. There is a corresponding slot in the piston which enables the connecting rod-wrist pin assembly to be inserted. The primary difficulty to this technique is that the wrist pin portion of the connecting rod-wrist pin assembly is not permitted to bear fully on the openings in the piston. Because a slot in the piston is necessary to permit insertion of the assembly, the wrist pin assembly bears only on the top and sides of the opening in the piston, rather than around the entire periphery of the wrist pin as in conventional designs. This presents a serious problem in low temperature compressors wherein the compression ratio is much higher and, consequently, the forces between the wrist pin and piston are high. It will be appreciated that the loss of part of the bearing surface will result in higher forces per unit area on the remaining bearing surfaces. Another difficulty is the complicated structure of the connecting rod and wrist pin assembly, which makes machining more difficult. Moreover, maintaining squareness of the connecting rod relative to the crankshaft and piston is much more difficult to achieve than in the case where the connecting rod is joined to the piston by a separate, cylindrical wrist pin.